Jack-in-the-box compounds

Contributed by Dr Sarah L. Masters, Department of Chemistry, University of Canterbury, Private Bag 4800, 

If radicals are formed in solution, what happens upon vaporization Around this time, studies of the gas-phase structures of the phosphorus and arsenic compounds were initiated in two separate gas electron diffraction (GED) laboratories. It was soon clear that in both cases the compounds were effectively 100% dissociated in the gas phase. However, determination of the molecular structures of the P[CH(SiMe3)2]2 and As[CH(SiMe3)2]2 radicals was a very difficult undertaking as in GED studies resolution of similar interatomic distances is difficult if not impossible. In P[CH(SiMe3)2]2 there are P—C distances and inner and outer Si—C distances, which are aU nearly the same, and there are similar problems with non-bonded distances, P Si, Si Si and C C. In As[CH(SiMe3)2]2 the problems are essentially the same. There are also distortions due to steric crowding, and the molecules have little symmetry (C2 in the gas phase). 

Nearly 20 years went by. Then the development of methods that used computed parameters to aid interpretation of GED data allowed the problem to be revisited. Solving the crystal structure of [P[CH(SiMe3)2]2 2 was straightforward. There was one molecule per asymmetric unit, with the four 

The structure of P2[CH(SiMe3)2]4 in the crystalline phase. Redrawn from [91] with permission of The Royal Society of Chemistry. 

Next came the crystal stmcture of As2[CH(SiMc3)2]4. There were eight independent molecules in the asymmetric unit [92]. Seven had the same conformation (labeled conformation 1), but the eighth was different (conformation 2), and was almost the same as crystalline P2[CH(SiMc3)2]4. This provided a rare opportunity to analyze the impact of the crystal environment on the molecular structures that were adopted, and to get some statistical data. 

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